Contra Rotating Axial Fans

Propeller for airplane, prop for boat, prop for helicopter...good for torque correction and stability and increased efficiency...

http://en.wikipedia.org/wiki/Contra-rotating_propellers

Several Aircarft manufactures (fixed wing and rotorcraft) use contra-rotating propellors as they eliminate rotational torque and increase power.

Helicopters with this system can lift heavier loads and fly faster, they also have the benefit of not requiring the additional complexity of having the tail rotor.

The V22 - Osprey uses this capability to mix the VTOL functionality of a Helo with the speed and range of a fixed wing aircraft.

V-22 OSPRAY Formidable Aircraft - YouTube

Chinooks use this to lift massive loads.

And Fixed wing aircraft have used them at various times to increase speed and manuverability.

In some processes, Ducted fans use contra-rotating fans to produce a laminar air flow. i.e. wind tunnels.

Regards,
Sapper

Expensive to run and operate… Still not very safe. It is the only craft that is not insulated in order to reduce weight..it cannot be used in place of multi-stage axial fans to boost up air only where a variable balance between the static and dynamic displacement of air is required such as in the case of Osprey.